SAB has never been acknowledged or discussed by Fuji, despite the fact that they have been fixing cameras that have the issue. Therefore, speculation about its cause or cure is baseless--i.e., citing the Mayan calendar as a reason for SAB has just as much validity as any of the other theories being floated. Bottom line: No. One. Knows. What we do know is that for a long time, there were many complaints posted about it here, and then the complaints stopped. All you read these days are FUD posts from those who don't even own a Fuji but apparently live in constant mortal fear of SAB. Read into that what you will.

2. Built-in flash - better (flash blends in better) than X-E1

I (somewhat) prefer the flash on my X100 to that on my X-E1, so I'd say that what you're suggesting here is already true. (Although you can bounce the X-E1 flash, which is a nice capability to have.) Since Fuji is making no claims about an improved flash on the X100s, informed speculation would suggest that it will be the same.

3. lens @23mm sharper than X-E1 zoom kit at the same aperture?

No one can say with certainty until the X100s is released, but there is a VERY HIGH probability that a high quality 23mm prime lens, tweaked specifically to its sensor, is going to produce sharper results than a high quality zoom at the same focal length. But it's not an absolute given... note that most reviews of the 18-55 conclude that it's better at 18mm than Fuji's prime lens of the same focal length.